Sensitivity analysis in the design of multi-MW wind turbine rotor blades

Nowadays blades are becoming a real determinant of the new generation wind turbines capital expenditure; longer blades are more cost intensive and a lot of care is thus needed to perform their design in the most suitable way. Research is now exploring new frontiers to develop beyond-state-of-art technologies and design philosophies with the purpose to face new limitations and to reduce the final cost of energy. Among these new strategies, this work investigates the possibility to use advanced materials, such as carbon fiber, to ensure an higher out-of-plane stiffness and a lower total mass and the effects of including the rotor radius within the set of active design variables to be optimized. These analyses have been carried out using an innovative free form methodology that considers the airfoils shapes as degrees of freedom within the optimization process, limiting the influence of the initial airfoils choice on the optimization output; indeed, in a recent work developed at Politecnico di Milano, the feasibility and effectiveness of the free form approach have been demonstrated and it has proved to be successful. Since the airfoils are not considered \textit{frozen} parameters, it is possible to get an idea of how the aforementioned optimization strategies have an impact on the choice of the optimal airfoils, thus extending the intrinsic aero-structural coupling typical of wind turbine blades optimization. To pursue the scopes of this work some important modifications turned out to be necessary to make the code properly deal with the new proposed analyses.